1. Field of the Invention
The present invention relates generally to fixation devices and, more particularly, to biologically compatible screws and matching drivers.
2. Description of Related Art
Since the beginning of recorded history, mankind has exhibited an insatiable appetite for constructing new devices and repairing broken ones. One meter for measuring the progress of a society through the generations has been the creativeness and craftsmanship of that societies"" architects and engineers.
From ancient wooden aqueducts of the Nile, to lightweight compositional structures of the Space Shuttle, to sophisticated artificial joints of modern medicine, a common ingredient has always been the fastener. Whether the fastener is threaded, removable, or integral with the structure, an accepted engineering principle is that the strength of a product is only as great as the product""s weakest link.
In the interest of promoting strength above all else, the prior art has primarily endeavored to construct metallic fasteners of varying sizes and shapes for the majority of applications. The strength of the metal fastener, however, is not achieved without costs. For many applications, metal can be relatively heavy, expensive, and subject to corrosion. Metal fasteners generally are not recyclable and, additionally, are neither biocompatible nor resorbable, when used in connection with medical applications.
Threaded, resorbable fasteners have existed in the prior art for medical applications, such as bone repair and regeneration. A typical resorbable fastener comprises a threaded shaft, a head, and an internal socket disposed within the head for accommodating a driver therein. This internal-socket fastener, although biocompatible and resorbable, has suffered from design deficiencies.
Since the prior art resorbable fastener is designed to be secured to bone within the human body, tissue or other debris may be introduced into the internal socket of the fastener. Materials introduced into the internal socket of the fastener can substantially attenuate or eliminate proper operability of the fastener. For example, a surgeon may have difficulty properly fitting a driver into the internal socket of a fastener that has become partially occluded with debris or human tissue. Since resorbable fasteners are generally manufactured having sizes on an order of millimeters, an obstruction of the very tiny internal socket of a fastener may be difficult to remove.
Although resorbable fasteners are inherently not as strong as metal fasteners with regard to rigidity, sheer strength, etc., resorbable fasteners offer very important benefits for medical applications, including biocompatability and resorbability. Since resorbable materials are relatively weak, as compared to the strength of metal, design considerations for resorbable fastener should maximize strength.
The specific internal-socket design of the prior art resorbable fastener does not appear to be particularly suited for medical and other applications where the strength of the non-magnetic fastener should be optimized. When a prior art internal-socket resorbable fastener is firmly secured into bone, for example, the driver may in some instances disrupt (strip) the internal socket of the fastener. Since the internal-socket is positioned along the rotational axis of the resorbable fastener, a very small moment arm must be utilized to rotate the fastener, resulting in the exertion by the driver of a relatively high rotational force onto the walls of the internal socket. The tiny construction of each internal-socket fastener (on the order of millimeters), the relatively weak material, and the relatively small application moment arm of the internal-socket fastener, all contribute to the relative sensitivity of the system. A need has thus existed in the prior art for a non-magnetic fastener having increased strength.
In additional to the limited strength associated with prior art non-magnetic fasteners, prior art non-magnetic fasteners have also suffered from relatively thick heads. As a result of the relatively weak material of the prior art internal-socket fastener, the head of the internal-socket fastener is typically manufactured to have a relatively thick dimension in a direction parallel to the rotational axis of the fastener. The thicker head of the prior art fastener provides a greater surface area for frictional application of torque by the driver upon insertion of the driver into the internal socket of the fastener. The relatively thick head of the prior art fastener, however, can undesirably protrude from the surface within which it is mounted, thus creating an undesirable non-flush surface.
As a result of the relatively small dimensions of the resorbable fasteners in the context of, for example, bone repair and regeneration applications, the tiny resorbable fasteners are prone to being improperly placed into the target structure. The prior art driver does not firmly hold the fastener and, accordingly, may not accurately align the axis of the fastener with the axis of the driver. The fastener can thus be inadvertently cross threaded or otherwise improperly secured within the target structure. Additionally, as a result of the relatively loose fit between the prior art fastener and driver, the fastener may become dislodged from the target structure and/or the driver, before being completely secured within the target structure.
Prior art drivers used to secure fasteners into target structures are typically not disposable. Accordingly, a single driver is used to secure a plurality of fasteners into the target structure or structures. The user is thus required to manually pick up and align each fastener with both the target structure and the driver, before the fastener can be secured within the target structures. Additionally, due in part to the relatively tiny dimensions of the fastener, a user may accidentally obtain a fastener, having a size other than the desired size, and attempt to secure the improperly-sized fastener into the target structure. The process of manipulating the fastener from the operating table into the target structure, accordingly, can be time consuming and subject to human error. In medical applications, the handling of the fastener by the hand of the user and, further, the multiple uses of the driver on a plurality of fasteners, can increase a probability of infection.
The high-torque fastener of the present invention includes a threaded shaft and a star-shaped head. In a preferred embodiment, the high-torque fastener comprises a resorbable or other non-magnetic material. A mating driver snugly fits around the star-shaped head of the fastener, to thereby apply torque to the perimeter of the star-shaped head. Since the high-torque fastener does not incorporate an internal socket, the high-torque fastener of the present invention is immune from the prior art problem of the interior socket becoming obstructed with tissue or other debris.
In contrast to the prior art internal socket fastener, the high-torque fastener of the present invention harnesses a relatively large moment arm. Sufficient torque is generated, via frictional contact between the driver the perimeter of the star-shaped head, without the introduction of excessive and potentially destructive frictional forces being introduced onto the star-shaped head. In other words, since the high-torque fastener of the present invention utilizes a relatively large moment arm, a relatively small rotational force can be used to apply relatively high torque to the fastener of the present invention.
The star-shaped head of the high-torque fastener provides a relatively large surface contact area for application of frictional rotational forces by the driver and, further, facilitates a uniform distribution of torque about the perimeter of the high-torque fastener of the present invention. The star-shaped head and the mating driver provides a system for applying relatively high rotational forces to the fastener to thereby firmly secure the fastener into a target structure. When removal of a resorbable, high-torque fastener is required, for example, the head of the high-torque fastener is not as susceptible to damage, compared to internal socket fasteners. The resulting high-torque fastener and driver combination of the present invention is more reliable and less subject to damage, compared to prior art internal socket systems, even when fasteners having sizes on the order of millimeters are used.
In accordance with one embodiment of the present invention, both the head of the high-torque fastener and the driver comprise tapered surfaces for providing a better frictional contact between the star-shaped head and the driver, when the target structure comprises a counter sunk surface or when the high-torque fastener is counter sunk. The enhanced frictional contact from the tapered surfaces can increase the application of torque to the star-shaped head by the driver, when the target structure comprises a counter sunk surface or when the high-torque fastener is counter sunk.
Although prior art internal socket fasteners required relatively thick heads for increasing the surface area between the internal socket and the driver, the high-torque fasteners of the present invention can be constructed having relatively thin heads. Since the head of the high-torque fastener of the present invention is less susceptible to damage by the driver, the head may be manufactured having smaller proportions. When the head and the driver are tapered, and the high-torque fastener is secured within a counter sunk hole of a target structure, for example, the profile of the fastener of the present invention is further reduced. Accordingly, the high-torque fastener of the present invention can be secured into a target structure in such a way as to make the high-torque fastener less palpable to a patient.
The high-torque fastener and driver combination of the present invention is constructed to facilitate fast and easy centering of the fastener on the driver. The surface of the head of the high-torque fastener of the present invention is slightly rounded, and the edge forming the opening of the driver has a slight taper corresponding in radius to that of the surface of the head of the high-torque fastener. This centering feature of the present invention can attenuate the occurrence of improper loading of the high-torque fastener within the driver.
In accordance with another aspect of the present invention, the high-torque fastener and the driver are configured to snugly fit together. When the high-torque fastener is secured within the driver, any shaking of the driver by the hand of a user cannot dislodge the high-torque fastener therefrom. When the high-torque fastener is secured within the driver, a user can grip only the high-torque fastener and rotate the high-torque fastener without touching the driver, so that the driver is completely below the high-torque fastener, without the high-torque fastener becoming dislodged from the driver. During the rotating action, the driver does not contact any object except for the high-torque fastener. In accordance with yet another aspect of the present invention, the high-torque fastener snaps into the driver to thereby provide a firm and secure fit. The strong hold of the driver on the high-torque fastener helps to ensure proper alignment of the rotational axes of the fastener and driver and, further, prevents the high-torque fastener from inadvertently being dislodged.
An automatic ejection mechanism is provided in accordance with yet another aspect of the present invention, for partially ejecting the high-torque fastener from the driver, as the high-torque fastener is secured within a target structure or structures, to thereby facilitate removal of the driver from around the high-torque fastener after the high-torque fastener is secured within the target structure or structures.
In accordance with still another aspect of the present invention, the driver is supplied with a high-torque fastener pre-secured therein, and is disposable. The tedious prior art process of selecting a properly-sized fastener, orienting the fastener, and subsequently assisting in the alignment of the fastener between the target structure and the driver as the driver is used to rotate the fastener, is reduced or eliminated. Time is saved, resulting in saved costs in the operating room, for example. Additionally, potential errors from the selection of the wrong-sized fastener, or an inappropriate alignment of the fastener relative to either of the target structure or the driver, can be reduced or eliminated. The disposable driver and fastener combination of the present invention offers added convenience and reliability. Moreover, since a user does not have to directly handle the fastener and since the disposable driver is only used once, any likelihood of infection can be attenuated. In accordance with yet another aspect of the present invention, the disposable driver and fastener combination is sold in a sterilized package.
The present invention, together with additional features and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying illustrative drawings.